The invention relates to fluorescent lamps of the so-called "rapid start" type. Such lamps and provided with thermal switches, responsive to cathode (filament) heat, for turning off the cathode heating current after starting and during lamp operation.
Rapid start fluorescent lamps are provided with cathode heating current, for heating the cathodes to electron-emitting temperature so that the lamps start quickly without damaging the electron-emitting material of the cathodes. The cathode heating consumes about one and one-half to two watts of electrical power per cathode. While the lamps are operating, the cathodes can provide adequate electron emission without the need for the supply of heating current to the cathodes. Accordingly, turning off the cathode heating current when the lamps are operating can save about three or four watts of electrical energy per lamp, resulting in considerable energy and money savings in lighting systems. In this regard, see U.S. Pat. No. 4,517,493.
U.S. Pat. Nos. 4,097,779 and 4,114,968 disclose rapid start fluorescent lamps provided with a thermal cutout switch near each cathode, and in electrical series with the associated cathode, for turning off the cathode current after the lamps start and while they are operating. These patents disclose U-shaped bimetal switches sealed in glass envelopes which are mounted near each cathode. After each cathode is heated sufficiently by the heating current heat from the cathode causes the nearby bimetal switch member to bend and open the current circuit to the cathode.
The manufacture of fluorescent lamps involves coating the tungsten cathode coils with an electron emission coating. After the lamps are assembled, the cathodes are "activated" by passing current through them to heat them. However, the cathode current cutout switches in the lamps will turn off the activation cathode heating current prior to complete activation of the cathodes. U.S. Pat. No. 4,114,968 solves this problem by connecting fuse wires across the thermal switch, for shorting the switch and permitting activation of the cathodes. The fuses are then "blown" (severed), by applying an electrical pulse through each of the series-connected fuses and cathodes. The fuses must be able to carry the cathode activation current and also be capable of being "blown" by a current pulse of insufficient strength to damage the cathode. Fuse timing is also important, since the fuse-blowing pulse must be applied while the thermal switch is in open condition so it will not short-circuit the pulse away from the fuse.
The quick start designs currently being marketed share a common problem: hot restarting. When the lamps are turned off after the switches have opened, a cool-off period is required to allow the switches to close, thus permitting current to flow through the cathode and restart the lamp. A series of tests were performed on rapid start lamps from various manufacturers to determine the restart time in a worst case scenario. The test consisted of operating the lamps for 20 minutes at an ambient temperature of 110.degree. F., which is the approximate temperature that the lamps experience in a standard four-lamp fixture. The lamps were then shut off for approximately two seconds and turned back on at 108 volts. The time that was required for the lamps to start was recorded. The times recorded for three major lamp manufacturers ranged from 52 to 68 seconds. All of these restart times are longer than would be considered acceptable by the consumer. The present invention is directed toward providing a rapid start lamp having a considerably shorter hot restart time.